But former Metropolitan Police murder squad detective Colin Sutton has now challenged that view with a sensational new theory of his own.

Sutton, who was the most senior detective at the scene on the day of the gruesome discovery almost four years ago, revealed he has lingering doubts about the code-breaker’s death.

His view chimes with that of Westminster coroner Dr Fiona Wilcox, who recorded two years ago that Mr Williams was unlawfully killed. Her verdict was later overturned.

Police broke into Mr Williams’s flat in Pimlico in London in August 2010, and found his decomposing remains in a red bag padlocked from the outside.

Mr Sutton has now voiced fears that the flat may well have been "cleaned" of all incriminating traces while any poison in Mr Williams’s system may have been impossible to detect by then.

“The first thing that struck me was how hot it was. It was August yet the heating was on full blast,” he said, recalling arriving on the scene on the day of the discovery.

“It made me think immediately that the heating could have been left on to accelerate the decomposition of Gareth's body.

“If he had been poisoned then the chemical compounds might have vanished by the time toxicology tests were conducted.”

He added: "There is also the possibility that something very unusual and hard to detect was used to poison Gareth. It may seem far fetched but we know that Alexander Litvinenko was killed four years earlier in London with a rare radioactive isotope.

“I remain convinced the flat was tidied up after his death. That may have been to protect national security - or it might have been something more sinister. If that is the case, then it could have been the perfect murder."

Mr Williams, from Valley in Anglesey, was a maths PhD graduate who began working at GCHQ in Cheltenham in 2001.

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The Editor

Mark Thoma

Liverpool-born Mark joined the Daily Post in January 2014 after seven years as editor of its Merseyside sister title the Liverpool Post. He started out as a weekly news reporter on Wirral Newspapers, and spent seven years at the Daily Post and Liverpool Echo. He was The Press Association's regional correspondent for North Wales, Merseyside and Cheshire from 1983 to 1997, before returning to the ECHO as deputy news editor. He has won a number of journalism awards, including the UK Press Gazzette Regional Reporter of the Year award, and in 1993 wrote a book on the James Bulger murder.